In the case of known domestic installations that are fitted with a PV system for providing an internal power supply, the PV system is designed in such a way that, as a maximum, it feeds as much electrical current into a line branch of the domestic installation that a current flowing in the line branch does not exceed a maximum permissible value. In order to ensure the energy supply even when the PV system does not generate and feed in sufficient electrical current, the line branch is connected to an external energy supply network. The connection to the external energy supply network usually takes place by way of an energy meter that is fitted with a line protection fuse. It is ensured by the line protection fuse that no more electrical current than is permissible for the line branch is fed into the line branch from the external energy supply network. The PV system, however, is not necessarily connected to the line branch by way of the energy meter with the line protection fuse, but at another location of the line branch, for example at its end in the region of the roof of the house. Since the current generated and fed in by the PV system then does not flow through the line protection fuse of the line branch concerned, the case may occur where more current than is permissible is fed into the line branch, i.e. the line branch is not protected by the line protection fuse from a state of overcurrent, in which the current flowing in the line branch exceeds the maximum permissible value.
The documents US 2011/0241426 A1 and, belonging to the same patent family, U.S. Pat. No. 8,362,646 B2 disclose a method by which states of overcurrent within a line branch with two separate current feeds are to be avoided. For this purpose, a current sensor, with which the current fed in from the external energy supply network is recorded, is provided in the region of the connection point for the external energy supply network. This value is transmitted to a control of an inverter of the PV system. It is then checked by the control whether the sum of a current fed in from the external energy supply network and a current fed in from the PV system exceeds the maximum permissible current value for the corresponding line branch. If the maximum permissible current value is exceeded, the PV system is switched off or cut back in such a way that the sum of the currents fed in does not exceed the permissible current value. Switching off of the PV system also takes place if the information that is required to check whether the sum of a current fed in from the external energy supply network and a current fed in from the PV system exceeds the maximum permissible current value for the corresponding line branch is not received. If the PV system does not respond to the commands for switching off or cutting back when the permissible current value is exceeded by the sum of the two current feeds, the current is also not switched off by a power protection fuse provided at the connection point for the external energy supply network.
The document EP 2 388 879 A1 discloses a different method, by which states of overcurrent within a line branch into which electrical current from an electrical energy supply network and electrical current from a local energy generating system are fed are to be avoided. A protective device is provided for this purpose in the region of the connection point for the external energy supply network. The protective device receives on the one hand a measured value of a current that is fed in from the external energy supply network, and on the other hand a measured value of a current that is fed in at the same point in time from the local energy generating system. A sum of the two measured values is determined by the protective device. If the sum of the currents fed in exceeds the maximum permissible value for the current in the line branch, a protective switch that separates the line branch from the external energy supply network is opened. The individual measured values are provided at the place where they are generated with a timestamp. Until the protective device has received all of the measured values associated with the same point in time, the measured values already available are buffer-stored.
Document U.S. Pat. No. 6,605,881 B2 discloses a connection box by way of which a PV system and an external energy supply network can be connected to a line branch with a load. Provided at the connection point for the line branch with the load is a current sensor, by way of which a current flowing to the load is detected. Also provided is a voltage sensor, with which the voltage of the external energy supply network is detected. From the two values it can be determined what power is being consumed at that moment by the load. The PV system is then controlled in dependence on the power required by the load, it being ensured that the power fed in from the external energy supply network does not exceed a predetermined value. To protect against a state of overcurrent, the connection box has a protective switch, with which the power branch can be separated from the PV system and the external energy supply network if the sum of the two currents is greater than a maximum permissible current of the line branch with the load.
The document WO 02/29954 A1 discloses the use of a controllable overcurrent protection for the connection of a load to an energy supply network. By allowing the overcurrent protection to be adapted for example with regard to its triggering threshold and its triggering characteristic, it is intended that the overcurrent protection can be used flexibly for various loads, allowing optimum protection from a state of overcurrent to be ensured in each case by taking their various requirements into account.